Effect of nanoclays on high and low temperature degradation of fluoroelastomers

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Polymer Degradation and Stability 93 (2008) 188e200www.elsevier.com/locate/polydegstab

Effect of nanoclays on high and low temperature degradationof fluoroelastomers

Madhuchhanda Maiti, Suman Mitra, Anil K. Bhowmick*

Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, West Bengal, India

Received 26 May 2007; received in revised form 5 August 2007; accepted 11 October 2007

Available online 22 October 2007

Abstract

High temperature degradation of a fluoroelastomer and its nanocomposites was carried out from room temperature to 700 �C using thermog-ravimetric analysis (TGA) in nitrogen and oxygen atmospheres. The presence of the unmodified nanoclay enhanced the onset of degradation inboth the environments, because of polymerefiller interaction, exfoliation, uniform dispersion and high thermal stability of the layered silicates.In the derivative curve, there was a single Tmax, indicating one-stage degradation for all the samples. The non-isothermal activation energy ofdegradation was determined using the Kissinger and the FlynneWalleOzawa methods. The nanocomposites showed higher activation energythan the neat elastomer. The activation energy of degradation, as observed by isothermal kinetics, was 165, 168 and 177 kJ mol�1 for the neatelastomer, modified and unmodified clay filled samples, respectively. Intrinsic viscosity, measured after low temperature ageing (125e175 �C)showed that the viscosity values were higher for the nanocomposites. The mechanism of degradation is discussed.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Fluoroelastomer; Nanoclay; Nanocomposite; Thermal degradation

1. Introduction

Polymereclay nanocomposites are one of the importantfields of modern research, because of the capability of impartingnew polymer properties to the base polymer with the addition ofvery small amount of filler (<10 wt%) [1]. Polymereclaynanocomposites exhibit increased mechanical and dynamicmechanical properties [2e5], ionic conductivity [6], gas per-meability [7], thermal stability and fire retardancy properties[8e10] relative to the virgin polymer. Although extensivestudies on thermal stability are available on thermoplasticeclay nanocomposites [8e21], there are no systematic studieson thermal degradation of elastomereclay nanocomposites.

The demand for high-performance elastomers like fluoroe-lastomers having high temperature and oil resistance is evergrowing. Fluoroelastomers are increasingly used as high-

* Corresponding author. Tel.: þ91 3222 283180; fax: þ91 3222 220312.

E-mail address: [email protected] (A.K. Bhowmick).

0141-3910/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2007.10.007

performance seal materials in many industrial and space appli-cations due to their excellent thermal, oil and chemical resis-tance arising from their CeF bonds. In order to furtherimprove the performance, the fluoroelastomer-nanocompositeswere synthesized and their mechanical, morphological andswelling behaviours were extensively reported in our previouscommunications [22e24]. It has been observed that even ata low loading of 4 phr of clay, these properties are significantlyenhanced.

Thermal degradation of fluoropolymers has been studied byseveral researchers [25e31]. However, to the best of our knowl-edge, no detailed studies exist on the thermal decomposition offluoroelastomer-nanocomposites. In this paper, the thermaldegradation behaviour of fluoroelastomer-nanocompositesboth in nitrogen and oxygen atmosphere and also at high andlow temperatures has been studied. The kinetics of the thermaldegradation of fluoroelastomer-nanocomposites was investi-gated using different kinetic methods. Thermogravimetric anal-ysis has been used widely to estimate the activation energies ofdegradation. The isothermal degradation was also studied.

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Table 1

Different formulations for fluoroelastomereclay nanocomposites and their

designation

Composition Designation

Viton B-50 F

Viton B-50þ 2 phr NA FNA2



189M. Maiti et al. / Polymer Degradation and Stability 93 (2008) 188e200

It was also the purpose of this work to study the degradation offluoroelastomer-nanocomposites and its mechanism in therange of 125e175 �C, which is usually the processing temper-ature of this elastomer. The low temperature degradation wasstudied with the help of changes in intrinsic viscosity.

2. Experimental


Viton B-50þ 4 phr 20A F20A4
2.1. Materials Viton B-50þ 3 phr DIAK #1 F-V
Viton B-50þ 4 phr NAþ 3 phr DIAK #1 FNA4-V



Viton B-50þ 4 phr 20Aþ 3 phr DIAK #1 F20A4-V

Viton B-50 (a terpolymer of vinylidene fluoride (VF2), hex-afluoropropylene (HFP) and tetrafluoroethylene (TFE), density1850 kg m�3 at 25 �C, 68% F, Mooney viscosity, ML1þ10 at120 �C¼ 39) was procured from DuPont Dow Elastomers,Freeport, Texas, USA. Unmodified sodium montmorillonite,Cloisite NAþ (NA) and organically modified clay, Cloisite20A (20A) were obtained from Southern Clay Products,Texas, USA. Methyl ethyl ketone was supplied by Nice Chem-icals Pvt. Ltd., Cochin, India. Hexamethylene diamine carba-mate (HMDC, DIAK #1) was supplied by NICCO CorporationLtd., Shyamnagar, India.

2.2. Preparation of fluoroelastomereclaynanocomposites

2.2.1. Unvulcanized samplesThe elastomer was first dissolved in methyl ethyl ketone

[20% (w/v) solution]. The clay was also dispersed in methylethyl ketone (1 g dispersed in 20 cm3). Then the appropriateamount of clay dispersion was added to the elastomer solutionand thoroughly stirred at room temperature for 3 h to make a ho-mogeneous mixture. The solution was then kept in air for 7days on a glass Petri dish to drive off the solvents completely.

2.2.2. Vulcanized samplesThe same procedure as described above was followed to pre-

pare elastomer-nanocomposites. HMDC (DIAK # 1) was thenmill-mixed with this elastomer in a proper proportion to makeit 3 phr (parts per hundred grams of rubber). The sampleswere then press-cured at 160 �C for 8 min in a hydraulic pressat 5 MPa pressure. The details of preparation of the sampleswere described in our earlier communication [22].

Table 1 reports various compositions used for this investi-gation and their designation.

2.3. Thermogravimetric analysis (TGA)

Dynamic thermogravimetric analyses were done using TAInstruments (Luken’s Drive, New Castle, DE, USA) TGAmodel Q 50. The samples (10� 2 mg) were heated from ambi-ent temperature to 700 �C in the furnace of the instrumentunder dynamic nitrogen and oxygen atmosphere flowing at60 ml/min. In order to study the kinetics of decomposition ofthe compounds, experiments were done at four heating rates,5, 10, 15 and 20 �C/min and the data of weight loss versus tem-perature were recorded online in the TA Instrument’s Q seriesExplorer software. The analysis of the thermogravimetric

(TG) and derivative thermogravimetric (DTG) curves wasdone using TA Instrument’s Universal Analysis 2000 softwareversion 3.3B. In the present study, the temperature correspond-ing to 5% degradation, taken as the degradation onset temper-ature (Tonset) and the temperature corresponding to themaximum value in the derivative thermogram (Tmax) were re-corded. The samples were run in triplicate and the averagevalues of Tonset and Tmax along with the standard deviationsare reported here.

2.4. Determination of activation energy of decomposition

2.4.1. FlynneWalleOzawa methodThe activation energy of decomposition was determined us-

ing TA Instruments’ Specialty Library software version 1.4.The analysis is based on ASTM method E 1641 [32], whichfollows the well-known FlynneWalleOzawa iso-conversionalmethod [33,34]. This iso-conversional integral method, sug-gested independently by Flynn and Wall [33] and Ozawa[34] uses Doyle’s approximation [35] of the temperature inte-gral. This method is based on the equation:

log b¼ logAE

RgðaÞ � 2:315� 0:457E

RTð1Þ

where b is the heating rate in �C/min; A, the pre-exponentialfactor; E, activation energy of decomposition in kJ/mole; R,the universal gas constant; a, the degree of conversion; T,the absolute temperature to reach the conversion and

gðaÞ ¼Za

0

da

f ðaÞ ð2Þ

is the integral conversion function.The procedure involved plotting the logarithm of b versus

(1/T ) for fixed values of conversion, a. For a constant a, theplot of log b versus (1/T ) should be a straight line whoseslope allows evaluation of the activation energy, E. To applythis iso-conversional method, heating rates of 5, 10, 15 and20 �C/min were chosen. In this study, we have used the con-version values of 20, 30, 40, 50, 60 and 70% which wouldgive a values 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7, respectively, for

190 M. Maiti et al. / Polymer Degradation and Stability 93 (2008) 188e200

the FlynneWalleOzawa method. The inputs for the automaticcalculation of the activation energy of decomposition usingthe software were the percent conversion and the temperaturelimit covering the major degradation steps in the thermogramsfor each heating rate.

2.4.2. Kissinger’s method [36]Kissinger’s method is one of the differential methods that

have been used by researchers to determine the activation en-ergy of solid state reactions from plots of the logarithm of theheating rate versus the inverse of the temperature at the max-imum reaction rate in constant heating rate experiments. Thebeauty of this method is that even without a precise knowledgeof the reaction mechanism and reaction order, the activationenergy can be determined using the following equation:

ln

�b

T2max

�¼�

lnAR

Eþ ln

�nð1� amaxÞn�1�� E

RTmax

ð3Þ

where b is the heating rate, Tmax is the temperature corre-sponding to the inflection point of the thermal degradationcurves which corresponds to the maximum rate, A is thepre-exponential factor, E is the activation energy, amax is themaximum conversion and n is the order of the reaction.From the slope of the plot of lnðb=T2

maxÞ versus 1000/Tmax,E can be calculated.

Both these methods were used in earlier experiments by ourgroup on different nanocomposites [37,38].

2.5. Low temperature ageing study

For the ageing study, representative unvulcanized sampleswere degraded in an air circulating ageing oven [Blue M Elec-tric Co., IL, USA] at 125, 150 and 175 �C for 12, 24 and 36 h,respectively. The samples were immediately weighed and dis-solved in MEK at 25 �C for further studies.

2.6. Intrinsic viscosity

NA

t)

Intrinsic viscosity was measured from the relative viscosityof polymer solutions measured at 25 �C using an Ostwald vis-cometer. The polymer was dissolved in methyl ethyl ketone.Five solution concentrations (0.1, 0.2, 0.3, 0.4 and 0.5%)were taken for the measurement.

ry U

ni

2.7. Fourier transform infra-red spectroscopy (FTIR)

FNA16

FNA8

ten

sity (A

rb

itra 20A

FTIR study was done in PerkineElmer Spectrum RX 1,UK, in the range 400e4000 cm�1 by casting a very thinfilm on a KBr pellet (w1% dispersion of nanocomposite inmethyl ethyl ketone). An average of 32 scans is reported here.

FNA4In

F20A4
2.8. X-ray diffraction studies (XRD)
2 4 6 8 102 , Degree

Fig. 1. XRD of different clays and their nanocomposites.

For the characterization of the rubber nanocomposites,XRD studies were performed using a PHILIPS X-PERTPRO diffractometer (The Netherlands) in the range of 2e10�

and Cu-target (l¼ 0.154 nm). The d-spacing of the clay

particles was calculated using Bragg’s law. The sampleswere placed vertically in front of the X-ray source. The detec-tor was moved at an angle of 2q while the sample was movedat an angle of q.

2.9. Transmission electron microscopy (TEM)

The nanocomposite samples for TEM analysis were preparedby ultra cryomicrotomy using Leica Ultracut UCT. Freshlysharpened glass knives with cutting edge of 45� were used toget cryosections of 100 nm thickness. Since these sampleswere elastomeric in nature, the sample temperature during ultracryomicrotomy was kept constant at e60 �C (which was wellbelow the glass transition temperature [Tg] of the rubber), atwhich the samples existed in hard glassy state, thus facilitatingultra cryomicrotomy. The cryosections were collected and di-rectly supported on a copper grid of 200-mesh size. The micros-copy was performed later using a transmission electronmicroscope (JEOL-2010, Japan, not attached to the cryomicrot-omy unit), operating at an accelerating voltage of 200 kV.

2.10. Swelling studies

Gel-fraction of rubber was calculated for the unvulcanizedsamples as

Gel-fraction ¼W2

W1

ð4Þ

where W1 and W2 are the weights of the sample before andafter extraction. The samples were extracted with methyl ethylketone for 24 h at 25 �C.

3. Results and discussion

3.1. Characterization of fluoroelastomer-nanocomposites using TEM and XRD

The XRD of different nanoclays and their nanocompositesis shown in Fig. 1. The pristine NA and 20A show (001)

Fig. 2. (a) Low and (b) high magnification TEM photographs of FNA4. (c) Low and (d) high magnification TEM images of F20A4. (e) Low and (f) high mag-

nification TEM images of FNA8. (g) Low and (h) high magnification TEM images of FNA16.



peaks at 7.5� [d001¼ 1.17 nm] and 3.6� [d001¼ 2.42 nm], re-spectively (Fig. 1). There is no peak in the X-ray diffracto-grams of FNA4 and F20A4, indicating exfoliated ordisordered intercalated morphologies at 4 phr of clay loading.It indicates that the clay tactoids are separated into platelets.In the case of FNA8, a small hump at lower angle (between6 and 7�) compared to that of pristine NA can be observed.FNA16 shows a peak at 6.6� [d001¼ 1.33 nm] and the claylayers are intercalated with larger d-spacing compared tothat of the pristine clay. Hence, at higher loading of clay,the polymer chains cannot exfoliate the clay layers.

As the XRD alone cannot determine morphology, we havealso carried out transmission electron microscopy to visualizethe structure of the nanoclays in the nanocomposites. The lowand high magnification TEM images of FNA4, F20A4, FNA8and FNA16 are shown in Fig. 2aeh. The low magnificationimage shows that the clay particles are well distributed inthe case of FNA4 (Fig. 2a). Some tactoids can be observedin the case of F20A4 (Fig. 2c). This may be due to the betterpolymerefiller interaction in the case of the unmodified clayfilled samples than the modified one (explained later withthe help of thermodynamics and surface energetics). Thehigh magnification image of FNA4 (Fig. 2b) shows that theclay layers are well separated, having single platelets of1 nm thickness. High magnification TEM image of F20A4also shows that the clay particles are mostly exfoliated alongwith some intercalation (Fig. 2d).

At higher filler loading, the clay particles are mostly stackedin the form of tactoids as evident from the low and highmagnification images of FNA8 and FNA16 (Fig. 2eeh). Theclay layers have an average thickness of 10 and 30 nm in thecase of FNA8 and FNA16, respectively. TEM and XRD resultsare in good accord. Hence, at higher filler loading only interca-lated morphologies are formed.

3.2. Effect of nanoclays on thermal degradationcharacteristics of fluoroelastomers

Typical TG curves and the corresponding derivative ther-mogravimetric (DTG) curves obtained at a heating rate of20 �C/min in nitrogen (N2) atmosphere for F, FNA4 and

0 200 400 600

0

20

40

60

80

100

Weig

ht, %

Temperature, °C

FFNA4F20A4

(a)

Fig. 3. Typical (a) TG and (b) DTG trace of diffe

F20A4 are shown in Fig. 3a and b. These TG curves corre-spond to a single-step degradation with well defined initialand final degradation temperatures and may be a result ofa random chain scission process. Tonset, Tmax, residue and max-imum rate of decomposition values for the above mentionedsamples are reported in Table 2. A shift of Tmax by 15 and11 �C to higher temperature can be observed in the case ofthe unmodified clay and the modified clay filled (4 phr) sam-ples, respectively. There is a significant reduction in the rateof decomposition in the presence of the unmodified clay atmajor degradation step (Fig. 3b).

In order to understand the effect of filler loading on thermalstability, the loading of the unmodified clay was varied from 2to 16 phr. The TG and DTG curves are shown in Fig. 4a and b.Tonset and Tmax along with the residue values are registered alsoin Table 2. As expected, an increase in filler loading increasesthe residue. Tonset and Tmax values gradually increase with in-creasing filler loading. An increment of 20 �C in Tmax valuecan be observed in the case of FNA16. Hence, with increasingfiller loading, the thermal stability increases. This is not a usualobservation in the case of nanocomposites. The previous reportson thermal stability of the nanocomposites show that the ther-mal stability is dependent on the morphology. Better the claydispersion, the higher is the thermal stability of the nanocompo-sites [8,20,37]. As the clay starts to agglomerate, the propertydeteriorates. But in the case of FNA8 and FNA16, despite inter-calated/aggregated structure, as evident from the XRD (Fig. 1)and TEM (Fig. 2eeh), the thermal stability increases. There-fore, it can be concluded that the thermal stability is not onlya function of clay dispersion and polymerefiller interaction,but also of clay content. Though the mechanical propertiesare related to dispersion of the clay in the polymer, the forma-tion of a barrier which hinders mass transport of degradingpolymer species and insulates the underlying polymer fromthe thermal source is responsible for higher thermal stability.Silicate layers also have higher thermal stability.

Typical TG curves and the corresponding DTG curves ob-tained at a heating rate of 20 �C/min in oxygen (O2) atmo-sphere for F, FNA4 and F20A4 are shown in Fig. 5a and b.Tonset and Tmax values for the above mentioned samplesare registered in Table 2. The TG curves exhibit one-step

0 200 400 600 800

0

1

2

3

4

5

Deriv. W

t, %

/°C

Temperature, °C

FFNA4F20A4

(b)

rent nanocomposites (unvulcanized) (in N2).

Table 2

Parameters obtained from the thermal degradation of different unvulcanized

nanocomposites in both N2 and O2 atmosphere

Sample Tmax, �C

(20 �C/min)

Tonset,�C

(20 �C/min)

Residues, % Maximum rate

of decomposition,

%/�C

F (in N2) 479� 1* 468� 1 0.57� 0.02 4.89� 0.15

FNA2 (in N2) 492� 1 474� 1 2.62� 0.08 3.49� 0.12

FNA4 (in N2) 494� 1 476� 1 4.27� 0.14 3.28� 0.13

FNA8 (in N2) 496� 1 478� 1 6.84� 0.27 3.12� 0.10

FNA16 (in N2) 499� 2 481� 2 14.54� 0.57 2.80� 0.09

F20A4 (in N2) 490� 1 469� 1 3.40� 0.12 3.35� 0.14

F (in O2) 433� 1 414� 1 0.68� 0.04 3.85� 0.15

FNA4 (in O2) 461� 2 441� 2 2.07� 0.08 2.99� 0.10

F20A4 (in O2) 418� 2 415� 2 1.69� 0.06 7.21� 0.26

*Standard deviation.


degradation with well defined initial and final degradationtemperatures. In O2, the overall thermal stability is poorerthan nitrogen atmosphere, as the O2 helps in chain scission.This is in line with the earlier observations on different elas-tomer systems [39]. A shift to the higher temperature in thepeak values of Tmax with the addition of 4 phr of the unmod-ified clay can be observed. There is 28 �C improvement inTmax value after addition of the unmodified sodium montmo-rillonite. Interestingly, in oxygen atmosphere, with the addi-tion of the modified clay, the Tmax value shifts to lowertemperature. A similar observation has been reported byother researchers on different nanocomposites with organo-clays [8,40].

The higher thermal stability of the unmodified clay filledsample in both nitrogen and oxygen atmospheres is due tobetter polymerefiller interaction in the case of the unmodifiedclay filled sample. It is due to polarepolar interactionbetween the unmodified clay and the polar fluoroelastomer.Polar hydroxyl groups of the unmodified clay attract thechains of fluoroelastomers having CdþeFd� bonds. In thecase of the modified clays, there may be some incompatibilitywith the matrix due to modification by long chain aliphaticamines. The better interaction in the case of the unmodified

100 200 300 400 500 600 700

0

20

40

60

80

100

350 400 450 500

30

60

90

Weig

ht, %

Temperature, °C

FFNA2FNA4FNA8FNA16

Temperature, °C

Weig

ht, %

FFNA2FNA4FNA8FNA16

(a)

Fig. 4. Typical (a) TG and (b) DTG curve

clay is also evident from higher gel-fraction of FNA4(3.41%) than that of F20A4 (2.19%). This can also be sup-ported by the morphology of different nanocomposites. TheTEM images show that the unmodified clays (Fig. 2b) aremostly exfoliated in the matrix compared to the modifiedone, where clays are mostly intercalated along with some ex-foliation (Fig. 2d). As there is better interaction between theunmodified clay and the fluoroelastomer, the elastomer chainsare anchored to the clay platelets at several points giving riseto shorter chain lengths. The better polymerefiller interactionin the case of the unmodified clay filled sample than the mod-ified one can be explained with the help of thermodynamics[22,23]. The free energy change of the system after mixingthe clay into the fluoroelastomers may be given as follows:

DGE ¼ DHE� TDSE; for elastomers ð5Þ

DGC ¼ DHC� TDSC; for clays ð6Þ

Therefore, total free energy change of the system is

DGS ¼ DHS� TDSS ¼ ðDHE þDHCÞ � TðDSE þDSCÞ ð7Þ

From the expression, DGS value will be negative and hence themost favourable interaction between the clay and the fluoroe-lastomer will take place when DHS is negative and DSS ispositive.

When polymer chains enter into the gallery of the clay, theyreside in a restrained form, i.e., DSE is negative. In contrast, theexpansion of the gallery by elastomer chains causes the entropychange in the clay, DSC to be positive. If the clays are exfoli-ated/intercalated, this may probably compensate the entropyloss associated with the confinement of elastomer chains.

Hence, DGS is mostly dependent on DHS value. DHS hasbeen calculated for different systems from the IR spectra(not shown here) using Fowkes’s equation [41],

DHS ¼ 0:236�Dn ð8Þ

DHS is �2.60 and 0.47 for FNA4 and F20A4, respectively. Asa result, the mixing of the unmodified clay with the

100 200 300 400 500 600 700

0

1

2

3

4

5

450 500 550

2

3

4

5

Deriv. w

t., %

/ °C

Temperature, °C

FFNA2FNA4FNA8FNA16

Deriv. W

t., %

/°C

Temperature,°C

FFNA2FNA4FNA8FNA16

(b)

showing the effect of filler loading.

0 200 400 600

0

20

40

60

80

100

Weig

ht, %

Temperature, °C

FFNA4F20A4

0 200 400 600

-2

0

2

4

6

8

Deriv. W

t., %

/°C

Temperature, °C

FFNA4F20A4

(a) (b)

Fig. 5. Typical (a) TG and (b) DTG trace of different unvulcanized nanocomposites (in O2).


fluoroelastomer will be more favourable than that of the mod-ified one. Hence, the better thermal stability of FNA4 thanF20A4 can be observed.

This can be also explained with the help of softehard acidbase theory and work of adhesion between clay and elastomer.In the unmodified clay, sodium ion is residing in the clay-gal-leries, which is a hard acid. On the other hand, there are ter-tiary ammonium ions in the modified clays, which are softacid in nature. Fluororubbers contain CdþeFd� bonds, whereFd� is a hard base in nature. So, it will be more attracted to-wards the hard acid [22]. Besides, the work of adhesion isalso higher in the case of the fluoroelastomer-unmodifiedclay system (67.63 mJ/m2) than that (51.42 mJ/m2) of fluoroe-lastomer-modified clay system [22]. It indicates better diffu-sion of the polymers into the unmodified clay interface [22].Hence, interaction will be more in the case of the unmodifiedclay.

3.3. Effect of curing on thermal degradation offluoroelastomer-nanocomposites

The TG curves of F-V, FNA4-V, F20A4-V, FNA8-V andFNA16-V at a heating rate of 20 �C/min in N2 atmosphere,

0 200 400 600

0

20

40

60

80

100

Weig

ht, %

Temperature, °C

F-VFNA4-VF20A4-VFNA8-VFNA16-V

(a)

Fig. 6. Typical (a) TG and (b) DTG trace of di

shown in Fig. 6a, also exhibit single-stage degradation withwell defined initial and final degradation temperatures. Thecorresponding DTG curves are shown in Fig. 6b. Tonset andTmax values of these samples are presented in Table 3. Withthe addition of only 4 phr NA and 20A, Tonset and Tmax valuesshift to higher temperature. A shift of 11 and 9 �C in Tmax canbe observed in the case of the unmodified clay and the modi-fied clay filled sample, respectively. Similarly, a shift of 11 and7 �C in Tonset can be observed in FNA4-V and F20A4-V com-pared to F-V. The thermal stability of the vulcanized samplesis lower compared to that of the unvulcanized one, in spite ofthe crosslinking due to the crosslinks formed by hexamethylenediamine carbamate. The bond energies for CeF, CeH, CeCand CeN are 485, 411, 346 and 305 kJ mol�1, respectively.As the CeN bond is weaker than CeC bonds, the crosslinkslower the thermal stability.

The effect of filler loading is much more prominent aftervulcanization. Tmax value shifts to higher temperature by 20and 39 �C in FNA8-V and FNA16-V, respectively.

Tonset and Tmax values of F-V, FNA4-V and F20A4-V in ox-ygen atmosphere are also reported in Table 3. Tonset and Tmax

values of FNA4-V are higher than those of F-V, but thermalstability of F20A4-V is poorer than F-V. The observation is

0 100 200 300 400 500 600 700

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Deriv. W

t., %

/°C

Temperature, °C

F-VF20A4-VFNA4-VFNA8-VFNA16-V

(b)

fferent vulcanized nanocomposites (in N2).

Table 3

Parameters obtained from the thermal degradation of different vulcanized

nanocomposites in N2 and O2 atmosphere

Sample Tmax, �C

(20 �C/min)

Tonset,�C

(20 �C/min)

Residues, % Maximum rate

of decomposition,

%/�C

F-V (in N2) 443� 1 423� 1 3.58� 0.11 2.66� 0.10

FNA4-V

(in N2)

454� 1 434� 1 7.16� 0.22 2.51� 0.10

FNA8-V

(in N2)

463� 2 441� 2 9.75� 0.35 2.50� 0.09

FNA16-V

(in N2)

482� 2 458� 2 17.95� 0.80 2.45� 0.08

F20A4-V

(in N2)

452� 1 430� 1 5.01� 0.20 2.58� 0.12

F-V (in O2) 480� 2 461� 2 0.34� 0.03 3.16� 0.13

FNA4-V

(in O2)

485� 2 469� 2 2.93� 0.12 3.07� 0.11

F20A4-V

(in O2)

473� 1 452� 1 1.77� 0.05 2.72� 0.09


similar to that of unvulcanized samples. The vulcanized sam-ples show better thermal stability than their unvulcanizedcounterpart in O2 due to formation of crosslinking.

3.4. Kinetics of degradation

Activation energy of degradation in nitrogen was calculatedfor a few selected samples from both non-isothermal and iso-thermal conditions. The non-isothermal kinetics was studiedusing differential Kissinger and integral FlynneWalleOzawamethods.

3.4.1. Non-isothermal conditionsUsing Kissinger’s method, the activation energies of the

decomposition of F, F20A4 and FNA4 in nitrogen atmospherewere calculated and the mean activation energy values aretabulated in Table 4. Typical Kissinger plots for F and FNA4are shown in Fig. 7a and b, respectively. Both the nanocompo-sites register higher activation energy of degradation (153 and162 kJ mol�1 for F20A4 and FNA4, respectively) than theneat elastomer (149 kJ mol�1). This implies that the thermal

Table 4

Activation energies and 60 min half-life temperature of different nanocomposites

Sample Non-isothermal method Iso

Average activation energy, kJ mol�1 Ra

(k0

Kissinger FlynneWalleOzawa 40

F 149� 4 145� 5 0.5

FNA4 162� 2 169� 3 0.3

F20A4 153� 2 155� 4 0.5

FNA8 165� 2 170� 4 e

FNA16 169� 2 173� 4 e

F-V 117� 2 124� 3 e

FNA4-V 126� 3 145� 4 eF20A4-V 119� 2 136� 3 e

stability of the nanocomposites is higher than that of neat elas-tomer. With increasing filler loading, the activation energy in-creases in N2 (165 and 169 kJ mol�1, respectively for FNA8and FNA16). This may be due to higher amount of silicatecontent at higher filler loadings.

The same trend can be observed after vulcanization also.The activation energy of F-V, FNA4-V and F20A4-V is 117,126 and 119 kJ mol�1, respectively. The activation energy islower than that of the unvulcanized sample.

The mean activation energies obtained using FlynneWalleOzawa method in nitrogen are also listed in Table 4 and typicalFlynneWalleOzawa plots for F and FNA4 are shown in Fig. 8aand b, respectively. Both the figures show that the fitting straightlines are nearly parallel and thus confirm the applicability ofthis method in the conversion range studied (20e70%). Similarplots have been obtained for the other nanocomposites (notshown here). A similar trend in the thermal stability of the sam-ples is reflected from both Kissinger and FlynneWalleOzawamethods. With the addition of the nanoclays the elastomerexhibits increase in activation energy. The activation energiesof F, F20A4, FNA4, FNA8 and FNA16 are 145, 155, 169,170 and 173 kJ mol�1, respectively.

Total activation energy of degradation of nanocomposites isa function of the activation energy of degradation of elastomerand filler and the energy to break the polymerefiller interac-tion. As the polymerefiller interaction is better in the caseof the polar unmodified clay and the polar fluoroelastomer,the activation energy is much higher in FNA4 than F20A4.The activation energy increases with increasing filler loading.As the clay starts to agglomerate at higher loading, there is notmuch improvement in polymerefiller interaction but at thesame time silicate content increases at higher filler loading.The thermal stability is a function of clay dispersion as wellas silicate content and in this case the latter one predominates.Thus, the activation energy increases with filler loading. Thetemperature corresponding to 60 min half-life also increasesin the case of the nanocomposites compared to the neat elas-tomer (Table 4). The increment is much prominent in thecase of the unmodified clay filled sample indicating betterthermal stability. With increasing filler loading, 60 min half-life increases gradually.

obtained from various kinetic methods in N2 atmosphere

thermal method Average 60 min half-life

temperature, �Cte constant

), mg/min

Average activation

energy, kJ mol�1

0 �C

5� 0.02 165� 4 382� 8

8� 0.01 177� 5 396� 6

0� 0.02 168� 4 387� 5

e 396� 5

e 405� 6

e 362� 5

e 366� 4

e 364� 4

1.33 1.34 1.35 1.36 1.37 1.38 1.39-10.4

-10.2

-10.0

-9.8

-9.6

-9.4

-9.2

ln

( /T

2

ma

x)

1000/Tmax

(K-1

)

r = 0.99

1.30 1.32 1.34 1.36 1.38-10.8

-10.4

-10.0

-9.6

-9.2

1000/Tmax (K-1

)

ln

( /T

2

max

)

r = 0.99(a) (b)

Fig. 7. Typical Kissinger plot from the experimental data at different heating rates for (a) F and (b) FNA4.


The trend observed in the case of activation energy and60 min half-life for the vulcanized samples are in tandemwith those of the unvulcanized ones, though the values aremuch lower. The activation of FNA4-V (145 kJ mol�1) andF20A4-V (136 kJ mol�1) is much higher than that of F-V(124 kJ mol�1). The 60 min half-life for F-V is also lowerthan that of FNA4-V and F20A4-V.

0.4

0.6

0.8

1.0

1.2

1.4

0.6

0.8

0.7

0.9

1.0

1.3

1.2

1.1

1.4

Lo

g [H

eatin

g R

ate (°C

/m

in

)]

Lo

g [H

eatin

g R

ate (°C

/m

in

)]

1.28 1.30 1.32 1.34

20.0%40.0%60.0%

20.0%40.0%60.0%

1.30 1.32 1.34 11/T x

1/T x

(b)

(a)

Fig. 8. Typical FlynneWalleOzawa

3.4.2. Isothermal conditions: 1e10% weight loss in neatelastomer (F) and its nanocomposite (FNA4)

Isothermal thermogravimetry, in which the weight loss isrecorded for a sample heated to a pre-determined constanttemperature, is carried out at three different temperatures(370, 400 and 430 �C) in nitrogen. Isothermal study of onlya few selective samples was done. Values of the rate constant,

1.36 1.38 1.40 1.42

30.0%50.0%70.0%

30.0%50.0%70.0%

.36 1.38 1.40 1.4210

3, K

-1

103, K

-1

plots for (a) F and (b) FNA4.

0 2 4 6 8 10 12 14

0

20

40

60

80

100

Weig

ht, %

Time, min

F, 370F, 400F, 430

0 2 4 6 8 10 12 14

20

40

60

80

100

Weig

ht, %

Time, min

FNA4, 370FNA4, 400FNA4, 430

(a) (b)

Fig. 9. Plot of percentage weight loss versus time of degradation of (a) F and (b) FNA4 in nitrogen at 370, 400 and 430 �C.


k0 at 400 �C are reported in Table 4. The rate constants are inthe same range as reported in the literature [39]. These valuesare calculated from the slope of the plots (Fig. 9a and b) ofweight loss versus time for zero-order kinetics using the fol-lowing equation:

Rate of weight loss¼ k0½concentration�0 ð9ÞThe rate constant of FNA4 is lower than that of F. The ac-

tivation energy for degradation was calculated from the Arrhe-nius plot and found to be 165, 168 and 177 kJ mol�1 for F,F20A4 and FNA4, respectively (Table 4). The reason issame as explained earlier.

3.5. Effect of nanoclay on low temperature degradationof fluoroelastomers

Ageing study was done in the temperature range 125e175 �C, in order to study the effect of nanoclay on the degra-dation at processing temperature. The change in weight withageing time and temperature is shown in Fig. 10a and b.The percentage weight change is much higher in the case ofF than FNA4. For example, after 36 h ageing at 175 �C, 4%weight change can be observed in the case of F whereas0.7% change can be noticed in the case of FNA4. It indicates

10 15 20 25 30 35 400

1

2

3

4 F, 175 F, 150 F, 125

Weig

ht C

han

ge,%

Time, h

(a)

Fig. 10. Change in weight after low tem

that the nanoclay imparts thermal stability. The change in in-trinsic viscosity of F and FNA4 with ageing time and temper-ature is depicted in Fig. 11a and b. The intrinsic viscositydecreases with increasing time and temperature. For the neatelastomer, at 125 �C, it is linear with ageing time. Viscositydecrement in F is much prominent than that in FNA4. For ex-ample, at 175 �C after 36 h of ageing the decrement in intrin-sic viscosity is 80 and 70% for F and FNA4, respectively.Similar observations have been made at other temperatures.This decrement in viscosity is mainly due to chain scission,which is restricted with the addition of nanoclay.

3.6. Mechanism of degradation

The thermal degradation mechanisms of fluoroelastomerscontaining both hydrogen and fluorine in the repeating unit,like the terpolymer (vinylidene fluoride-co-hexafluoropropy-lene-co-tetrafluoroethylene) are so complex that at best it ispossible to have a meager comprehension of the processesand they can be interpreted in terms of two competing net pro-cesses [42]:

(1) degradation of the main chain carbon bonds, i.e. mainchain scission and

Time, h

10 15 20 25 30 35 400

1

2

3

4FNA4, 175FNA4, 150FNA4, 125

Weig

ht C

han

ge,%

(b)

perature ageing (a) F and (b) FNA4.

0 5 10 15 20 25 30 350.0

0.5

1.0

1.5

2.0

2.5F, 125F, 150F, 175

In

trin

sic V

isco

sity, d

Lg

-1

Ageing Time, h

0 5 10 15 20 25 30 350.0

0.5

1.0

1.5

2.0

2.5

In

trin

sic V

isco

sity, d

Lg

-1

Ageing Time, h

FNA4, 125FNA4, 150FNA4, 175

(a) (b)

Fig. 11. Plot of intrinsic viscosity of (a) neat elastomer and (b) nanocomposite after ageing in air oven versus ageing time.


(2) degradation by the splitting off of adjacent hydrogen andfluorine as hydrogen fluoride generating double bonds,as shown in Scheme 1.

If the polymer decomposes entirely by the process K1, noresidue would be expected. The process includes all variationsof mechanism involving random initiation of decomposition ofpolymer molecules to radicals, propagation of the macro rad-icals produced on decomposition, transfer (intermolecular) ofthe radicals and termination (by disproportionation and cou-pling) of the radicals to dead molecules. On the other hand,if the polymer decomposes by the process K2, there wouldbe a finite residue. The residue after TGA experiment inboth nitrogen and oxygen atmosphere for neat fluoroelastomer(F) is negligible and for nanocomposites (FNA4 and F20A4)the residue is the amount of filler content. Hence, it can besaid that K1 mechanism is dominating here.

In order to verify the degradation of the fluoroelastomerand its nanocomposites, FTIR spectroscopy of the pure fluo-roelastomer was done after high temperature (430 �C) isother-mal TGA experiment in nitrogen and after low temperature(175 �C) ageing in an air oven. Both the processes give similartype of IR spectra (shown in Fig. 12a and b) for fluoroelasto-mer. Hence, the mechanism does not change according to en-vironment or temperature. The IR spectra of FNA4 afterdegradation are also similar to those of F (illustrated inFig. 12c and d). Thus, the degradation mechanism remainssame even after addition of nanoclays. It can be observedfrom the spectra that transmittances due to CeF functionalgroups at around 1397, 1197, 900 and 504 cm�1 decrease in

-(CH2-CF2-CF2-CF-CF2-CF2-)-

-CH2-CF2 + CF2-CF(CF3)-CF2-CF2-. .

HF+ -(CH=CF-CF2-CF-CF2-CF2)-

CF3

CF3

K2K1

Scheme 1. Mechanism of fluoroelastomer degradation.

their intensities on increasing the time both at 175 and430 �C. The intensities are even less at 430 �C. There is a smallhump observed around 1620 cm�1, which hardly changes withtime and temperature. The remarkable decrease in intensitiesof the peaks for CeF group again indicates that the neat elas-tomer and its nanocomposites mainly degrade through mainchain scission, the degradation hardly involves eliminationof HF as shown by a marginal variation of the intensity of dou-ble bond with increasing time or temperature. After ageing ofneat elastomer, there is no gel-content. This also supports thatdouble bond formation is not predominating. Now the break-age of chain carbon atoms can involve the scission of CeFbonds both in the side group as well as on the backbone[30]. These can involve the formation of volatiles like fluoro-form (degradation of the side group) usually mixed with vinyl-idene fluoride fraction (scission of main chain).

The change in 1397 cm�1 peak (due to CeF bond) intensitywith time at 430 �C for F and FNA4 is shown in Fig. 12e. Thechange observed in transmittance of this peak is more intensein the case of F than FNA4. Once again, it proves the betterthermal stability of nanocomposite over the neat elastomer.

4. Conclusions

The effect of nanoclays on the thermal degradation of fluo-roelastomers both in nitrogen and oxygen atmosphere has beenstudied in low and high temperature regions. In general, thethermal stability is poorer in oxygen than in nitrogen. The un-vulcanized samples exhibit better thermal stabilities than thevulcanized ones in N2. The trend is reverse in O2 atmosphere.Two non-isothermal kinetics methods namely, Kissinger andFlynneWalleOzawa and isothermal kinetics methods havebeen used to study the thermal degradation of fluoroelasto-mereclay nanocomposites. The activation energies increasewith the addition of nanoclays. The activation energy of theunmodified clay filled sample (169 kJ mol�1) is higher thanthose of neat (145 kJ mol�1) and the modified clay filled sam-ples (155 kJ mol�1) as calculated from FlynneWalleOzawamethod. The intrinsic viscosity decreases with low tempera-ture ageing. The decrease is least for the unmodified clay filled

2000 1600 1200 800 400

40

60

80

Wave Number, cm-1

Tran

sm

ittan

ce, %

FF, 12, 175F, 24, 175F, 36, 1751197

1398

1620

900 505

2000 1600 1200 800 400

40

60

80

FF, 5, 430F, 10, 430F, 15, 430

Tran

sm

ittan

ce, %

Wave Number, cm-1

1197

1398

1627900 505

(a) (b)

2000 1600 1200 800 40020

40

60

80

Wave Number, cm-1

Tran

sm

ittan

ce, %

FNA4FNA4, 12, 175FNA4, 24, 175FNA4, 36, 175 1186

1398

893505

2000 1600 1200 800 400

40

60

80

FNA4FNA4, 15, 430FNA4, 10, 430FNA4, 5, 430

Wave Number, cm-1

Tran

sm

ittan

ce, %

1186

1398

1607

893 505

(c) (d)

(e)

5 10 15

35

36

37

38

39

40

Time, min

Ch

an

ge in

in

ten

sity, %

FFNA4

Fig. 12. IR of F after ageing at (a) 175 �C for 12, 24 and 36 h; (b) 430 �C in nitrogen for 5, 10 and 15 min. IR of FNA4 after ageing at (c) 175 �C for 12, 24 and

36 h; (d) 430 �C in nitrogen for 5, 10 and 15 min. (e) Change in intensity of 1397 cm�1 peak after heating at 430 �C in nitrogen for 5, 10 and 15 min.


system. The degradation mechanism of neat elastomer and thenanocomposites is same as observed from the FTIRspectroscopy.

References

[1] Pinnavaia TJ, Beall GW. Polymereclay nanocomposites. In: Wiley series

in polymer science. Chichester: Wiley; 2000.

[2] Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T,

et al. Mechanical properties of nylon 6-clay hybrid. J Mater Res

1993;8(5):1185e9.

[3] Lan T, Pinnavaia TJ. Clay-reinforced epoxy nanocomposites. Chem

Mater 1994;6(12):2216e9.

[4] Sadhu S, Bhowmick AK. Effect of nanoclay on the dynamic mechanical

properties of styrene butadiene and acrylonitrile butadiene rubber vulca-

nizates. Rubber Chem Technol 2005;78(2):321e35.

[5] Maiti M, Sadhu S, Bhowmick AK. Brominated poly(isobutylene-

co-para-methylstyrene) (BIMS)eclay nanocomposites: synthesis and

characterization. J Polym Sci Polym Phys 2004;42(24):4489e502.

[6] Vaia RA, Vasudevan S, Krawice W, Scanlon LG, Giannelis EP. New

polymer electrolyte nanocomposites e melt intercalation of poly(ethyl-

ene oxide) in mica-type silicates. Adv Mater 1995;7(2):154e6.

[7] Tsou AH, Measmer MB. Dispersion of layered organosilicates in

isobutylene-based elastomers. Rubber Chem Technol 2006;79(2):

281e306.

[8] Gu A, Liang G. Thermal degradation behaviour and kinetic analysis of

epoxy/montmorillonite nanocomposites. Polym Degrad Stab 2003;

80(2):383e91.

[9] Bourbigot S, Gilman JW, Wilkie CA. Kinetic analysis of the thermal deg-

radation of polystyreneemontmorillonite nanocomposite. Polym Degrad

Stab 2004;84(3):483e92.

[10] Jang BN, Wilkie CA. The thermal degradation of polystyrene nanocom-

posite. Polymer 2005;46(9):2933e42.


[11] Krump H, Luyt AS, Hudec I. Effect of different modified clays on the

thermal and physical properties of polypropyleneemontmorillonite

nanocomposites. Mater Lett 2006;60(23):2877e80.

[12] Modesti M, Lorenzetti A, Bon D, Besco S. Thermal behaviour of com-

patibilised polypropylene nanocomposite: effect of processing condi-

tions. Polym Degrad Stab 2006;91(4):672e80.

[13] Pandey JK, Reddy KR, Kumar AP, Singh RP. An overview on the degrad-

ability of polymer nanocomposites. Polym Degrad Stab 2005;88(2):

234e50.

[14] Paul M-A, Alexandre M, Degee P, Henrist C, Rulmont A, Dubois P. New

nanocomposite materials based on plasticized poly(L-lactide) and

organo-modified montmorillonites: thermal and morphological study.

Polymer 2003;44(2):443e50.

[15] Pramoda KP, Liu T, Liu Z, He C, Sue H. Thermal degradation behaviour of

polyamide 6/clay nanocomposites. Polym Degrad Stab 2003;81(1):47e56.

[16] Wang J, Du J, Zhu J, Wilkie CA. An XPS study of the thermal degrada-

tion and flame retardant mechanism of polystyreneeclay nanocompo-

sites. Polym Degrad Stab 2002;77(2):249e52.

[17] Gilman JW. Flammability and thermal stability studies of polymer lay-

ered-silicate (clay) nanocomposites. Appl Clay Sci 1999;15(1e2):31e49.

[18] Qin H, Zhang S, Zhao C, Yang M. Zero-order kinetics of the thermal

degradation of polypropylene/clay nanocomposites. J Polym Sci Part B

Polym Phys 2005;43(24):3713e9.

[19] Morgan AB. Flame retarded polymer layered silicate nanocomposites:

a review of commercial and open literature systems. Polym Adv Technol

2006;17(4):206e17.

[20] Costache MC, Jiang DD, Wilkie CA. Thermal degradation of ethyleneevinyl acetate copolymer nanocomposites. Polymer 2005;46(18):6947e58.

[21] Costache MC, Wang D, Heidecker MJ, Manias E, Wilkie CA. The ther-

mal degradation of poly(methyl methacrylate) nanocomposites with

montmorillonite, layered double hydroxides and carbon nanotubes.

Polym Adv Technol 2006;17(4):272e80.

[22] Maiti M, Bhowmick AK. Structure and properties of some novel fluoroe-

lastomer/clay nanocomposites with special reference to their interaction.

J Polym Sci Part B Polym Phys 2006;44(1):162e76.

[23] Maiti M, Bhowmick AK. New insights into rubbereclay nanocomposites

by AFM imaging. Polymer 2006;47(17):6156e66.

[24] Maiti M, Bhowmick AK. Effect of polymereclay interaction on solvent

transport behaviour of fluoroelastomereclay nanocomposites and predic-

tion of aspect ratio of nanoclay. J Appl Polym Sci 2007;105(2):435e45.

[25] Kader MA, Bhowmick AK. Thermal ageing, degradation and swelling of

acrylate rubber, fluororubber and their blends containing polyfunctional

acrylates. Polym Degrad Stab 2003;79(2):283e95.

[26] Zulfiqar S, Zulfiqar M, Rizvi M, Munirt A, McNeili IC. Study of the ther-

mal degradation of poly(chlorotrifluoroethylene), poly(vinylidene fluo-

ride) and copolymers of chlorotrifluoroethylene and vinylidene

fluoride. Polym Degrad Stab 1994;43(3):423e30.

[27] Baker Jr BB, Kasprzak DJ. Thermal degradation of commercial fluoro-

polymers in air. Polym Degrad Stab 1993;42(2):181e8.

[28] Morisaki S. Simultaneous thermogravimetryemass spectrometry and py-

rolysis e gas chromatography of fluorocarbon polymers. Thermochim

Acta 1978;25(2):171e83.

[29] Bartko J, Siemon Jr JT. Synergistic effects in fluorocarbon elastomer

seals produced by a combined temperature and radiation environment.

J Appl Polym Sci 1991;42(6):1739e44.

[30] Banik I, Bhowmick AK, Raghavan SV, Majali AB, Tikku VK. Polym De-

grad Stab 1999;63(3):413e21.

[31] Knight GJ, Wright WW. Thermal stability of some heat-resistant elasto-

mers. British Polym J 1989;21(3):199e204.

[32] Standard test method for decomposition kinetics by thermogravimetry,

ASTM E 1641-04.

[33] Flynn JH, Wall LA. A quick, direct method for the determination of

activation energy from thermogravimetric data. Polym Lett 1966;4(5):

323e8.

[34] Ozawa T. A new method of analyzing thermogravimetric data. Bull

Chem Soc Jpn 1965;38(11):1881e6.

[35] Doyle CD. Estimating isothermal life from thermogravimetric data. J

Appl Polym Sci 1962;6(24):639e42.

[36] Kissinger HE. Reaction kinetics in differential thermal analysis. Anal

Chem 1957;29(11):1702e6.

[37] Sadhu S. Ph.D. thesis. Indian Institute of Technology Kharagpur; 2005.

[38] Sengupta R, Sabharwal S, Bhowmick AK, Chaki TK. Thermogravimetric

studies on polyamide-6,6 modified by electron beam irradiation and by

nanofillers. Polym Degrad Stab 2006;91(6):1311e8.

[39] Bhowmick AK, Rampalli S, Gallagher K, Seeger R, Mcintyre D. The

degradation of guayule rubber and the effect of resin components

on degradation at high temperature. J Appl Polym Sci 1987;33(4):

1125e39.

[40] Duquesne S, Jama C, Le Bras M, Delobel R, Recourt P,

Gloaguen JM. Elaboration of EVA-nanoclay systems-characterization,

thermal behaviour and fire performance. Compos Sci Technol

2003;63(8):1141e8.

[41] Fowkes FM, Tischler DO, Wolfe JA, Lannigan LA, Ademu-John CM,

Halliwell MJ. Acidebase complexes of polymers. J Polym Sci Part A

Polym Chem 1984;22(3):547e66.

[42] Wall LA. Fluoropolymers. New York: Wiley Interscience; 1972.

Effect of nanoclays on high and low temperature degradation of fluoroelastomers

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